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I'm adding a lab to my course, unfortunately at the last minute. I'm going to have them work on a single-sided biopotential amp, working off a 5V supply. I guessed at parts I'd need, and got LM285 2.5V shunt voltage refs, and now I have to figure out how to use them.

I'll be using them to handle Vref on AD623 instrumentation amps, and as a virtual ground for filters and subsequent gain stages. For the 623 Vref needs, I figured on buffering the LM285 with a follower (100mA capable output on op amp), but I'm not sure I need to. I suppose I can do the same thing for the virtual ground point.

Does this approach sound OK? How would I choose a bias resistor for the LM285?

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  • \$\begingroup\$ Your link doesn't go where you think it does ... \$\endgroup\$
    – brhans
    Mar 19, 2015 at 13:21

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Turns out to be dirt simple. You put a bias on it calculated so that any additional currents going through it don't take is outside of the operating range. For example, when using it as a Vref input for an AD623, the source impedance is 20 kohms, and when powered off of 5 volts, the maximum current injection is 2.5V/20 kohms in either direction. or 0.125 ma. When using it as a virtual ground on an inverting op amp, we're talking about bias currents. The LM285 wants at least 10microamps through it, and can do 20 milliamps, so placing a bias of 500 microamps would put it in its operating range, and when using it for the AD623 Vref and a few inverting amps, there's nothing you can throw at this reference to bring it below 10 microamps of operating current, so you're good to go.

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